1. Field of the Invention
The present invention relates to a substrate obtained by filling through holes in a sintered product of aluminum nitride with an electrically conducting layer, to a process for producing the same, and to a metallized substrate having electrically conducting patterns formed on the opposing two surfaces of the substrate and in which at least portions of the electrically conducting patterns on both surfaces are electrically connected to each other through the electrically conducting layer filled in the through holes. More particularly, the invention relates to a process for producing a sintered product of aluminum nitride having a high thermal conductivity, exhibiting favorable and intimate adhesion between the electrically conducting layer filled in the through holes and the sintered product of aluminum nitride, without developing cracks in the electrically conducting layer.
2. Prior Art
A sintered product of aluminum nitride exhibits excellent properties such as a high thermal conductivity, favorable electrically insulating property and a coefficient of thermal expansion comparable to that of silicon (Si) for forming integrated circuits, and has, hence, been used as a substrate for semiconductor circuit components. A metallized substrate of aluminum nitride has electrically conducting patterns such as metal layers and thin resistance films formed on the opposing surfaces of the sintered product and in which at least portions of the electrically conducting patterns present on both surfaces are electrically connected together through the so-called via-holes formed by filling through holes in the sintered product with an electrically conducting layer. This metallized substrate of aluminum nitride is used for a substrate such as a substrate mounting a semiconductor that is electrically connected to an external semiconductor circuit through the via-holes.
A co-firing method has been employed as a method of forming the via-holes. According to the co-firing method, the firing of the electrically conducting layer and the sintering of the substrate are simultaneously executed through one time of firing, offering an advantage in that the number of the steps can be decreased compared to the method according to which the electrically conducting layer is fired after the substrate is fired. According to the conventional method of simultaneously sintering the aluminum nitride, however, the obtained sintered product exhibits a thermal conductivity of about 170 W/mK at 25xc2x0 C. due to limitation stemming from the firing of the electrically conducting layer that is effected simultaneously with the sintering of the substrate.
On the other hand, a two-step firing method has been carried out in order to fire aluminum nitride (simple substance) without having an electrically conducting layer (Japanese Unexamined Patent Publication (Kokai) No. 105525/1993). This method makes it possible to obtain a sintered product of aluminum nitride having a thermal conductivity which is as high as about 200 W/mK at 25xc2x0 C.
When the sintered product of aluminum nitride having via-holes and the metallized substrate is prepared according to the above-mentioned two-step firing method, however, it is difficult to obtain a sufficiently high adhesion strength between the sintered product of aluminum nitride and the electrically conducting layer forming the via-holes. Besides, cracks develop in the substrate of aluminum nitride or in the via-holes, resulting in an increase in the resistance in the via-holes and between the via-holes and the electrically conducting pattern, and a decrease in the adhesion strength between the via-holes and the thin film formed on the via-holes. Moreover, the substrate is warped to a large extent.
It has therefore been desired to develop a metallized substrate of aluminum nitride having a high thermal conductivity, exhibiting excellent adhesion strength between the electrically conducting layer forming the via-holes and the sintered product of aluminum nitride, without developing cracks in the sintered product of aluminum nitride or in the via-holes, and exhibiting a large adhesion strength between the electrically conducting pattern and the via-holes.
The present inventors have forwarded the study in order to solve the above-mentioned problems, and have discovered the fact that cracks develop in the substrate and in the via-holes and the substrate is warped by an increased amount due to the action of carbon after the dewaxing, that development of cracks in the substrate of aluminum nitride and in the via-holes and the warping of the substrate are suppressed by suppressing the content of residual carbon in the molded article after dewaxed to lie within a predetermined range, that the adhesion strength between the sintered product of aluminum nitride and the electrically conducting layer forming the via-holes can be sufficiently increased and stabilized by controlling the content of residual carbon in the dewaxed product, amount of addition of the aluminum nitride powder to the electrically conducting paste, and the temperature ranges of the two-step firing method to lie within particular ranges, that the thermal conductivity of the sintered product of aluminum nitride having via-holes can be sufficiently enhanced, and have thus arrived at the present invention.
That is, the present invention is concerned with a substrate obtained by filling through holes in a sintered product of aluminum nitride with an electrically conducting layer, wherein said sintered product of aluminum nitride has a thermal conductivity of not smaller than 190 W/mK, and the adhesion strength between said sintered product of aluminum nitride and said electrically conducting layer is not smaller than 5.0 kg/mm2.
The invention is further concerned with a process for producing a substrate by filling through holes in a molded article of aluminum nitride comprising an aluminum nitride powder, a sintering assistant and an organic binder, with an electrically conducting paste comprising 100 parts by weight of a refractory metal powder and 2 to 10 parts by weight of an aluminum nitride powder, dewaxing the molded article of aluminum nitride so that the content of residual carbon therein is within a range of from 800 to 3000 ppm, firing the molded article of aluminum nitride at a temperature of from 1200 to 1700xc2x0 C. and, then, at a temperature of from 1800 to 1950xc2x0 C.
Furthermore, the present invention is concerned with a substrate obtained by filling through holes in the sintered product of aluminum nitride with an electrically conducting layer, wherein said sintered product of aluminum nitride has a thermal conductivity of not smaller than 190 W/mK, the adhesion strength between said sintered product of aluminum nitride and said electrically conducting layer is not smaller than 5.0 kg/mm2, electrically conducting patterns are formed on both opposing surfaces of said substrate, and at least portions of the electrically conducting patterns on said both surfaces are electrically connected together through said electrically conducting layer.